For the purpose of removing substances usually eliminated with urine and for the purpose of withdrawing fluid, use is made of various methods for machine-aided blood treatment in acute or chronic kidney failure. In the case of haemodialysis (HD), a patient's blood is conveyed in an extracorporeal blood circuit through one chamber of a dialyser divided by a semipermeable membrane into two chambers, while a dialysing fluid flows through the other chamber. Both a convective and a diffusive substance exchange takes place via the membrane of the dialyser. Only a convective substance exchange is present in the case of haemofiltration (HF). Haemodiafiltration (HDF) is a combination of the two methods.
Known apparatuses for haemodiafiltration have one or more substitution pumps with which physiological fluid is fed to the patient's blood, while fluid is withdrawn via the dialyser or filter of the haemodiafiltration apparatus. The physiological fluid can be fed upstream or downstream of the dialyser to the arterial or venous branch of the extracorporeal circuit. The substitution of the fluid before entry of the blood into the dialyser or filter is referred to as pre-dilution and the substitution after exit of the blood from the dialyser or filter is referred to as post-dilution.
It has been shown that an HDF blood treatment in which a post-dilution takes place has a greater efficiency, with the same substitution delivery quantity, than a treatment in which a pre-dilution takes place. The greater cleaning capacity with the post-dilutive addition of substitution fluid compared with the pre-dilutive substitution can be traced back to the fact that the filtrate is obtained completely from the blood to be cleaned in the case of post-dilution, whereas in the case of pre-dilution the blood diluted with substitution fluid flows into the dialyser or filter before urea toxicants can be taken up during the flow through the patient.
A drawback with pre-dilution lies in the fact that an excessively high ultrafiltration rate, i.e. an excessively great withdrawal of fluid via the membrane of the dialyser or filter, leads to a thickening of the blood and an increase in the flow resistance in the dialyser or filter.
It has been shown that, with raised flow resistance, blood treatment apparatuses are no longer in a position to convey the blood to be cleaned at the set delivery rate, as a result of which the effectiveness of the blood treatment is reduced. The flow resistance in the dialyser or filter can, however, also lead to complete blocking-up of the membrane. The treatment is then interrupted, whereby the whole blood hose system possibly has to be replaced.
The flow resistance in the dialyser or filter on the blood side is dependent on the hematocrit of the blood, the properties of the membrane of the dialyser or filter and the ratio of the delivery quantities from blood pump and substitution pump. An increase in the flow resistance leads to an increase in pressure between the blood pump arranged upstream of the dialyser or filter and the dialyser or filter. Since both the hematocrit of the blood and the membrane properties change in the course of the treatment, a substitution adapted to the conditions is sought in order to increase the efficiency of the treatment.
Existing controls for the substituate delivery are based on a fixed ratio of the delivery rates of the blood pump and the substituate pump. There is known from European patent document EP 1 175 917 A1 a haemodialysis apparatus, wherein the control of two substituate pumps in pre- and/or post-dilute substitution takes place on the basis of the change in the transmembrane pressure or the hematocrit. In order to determine the transmembrane pressure, European patent document EP 1 175 917 A1 proposes that the pressure be measured both in the extracorporeal circuit and also in the dialysing fluid system.
German patent document DE 38 06 248 A1 describes a protective system for monitoring the pressure of the fluid circuit of a medical apparatus, wherein not only the static pressure, but also pressure fluctuations present in the fluid circuit are evaluated. German patent document DE 38 06 248 A1 proposes picking up the phase shift of pressure pulses, which are detected with a pressure sensor, in order to detect interruptions of the flow in the fluid circuit.
A method is known from U.S. Patent Application No. 2002/0174721 A1 for the detection of stenoses in a hose line system during an extracorporeal blood treatment. In order to detect a stenosis, the frequency spectrum of an oscillating pressure signal propagated in the extracorporeal blood circuit is analysed. It is concluded that there is a stenosis when the attenuation of at least one harmonic of the oscillating pressure signal changes.